This invention relates to insulated electrical bushings and more particularly to sealing systems for electrical conductors of insulated electrical bushings.
An insulated electrical bushing is used in an electrical device, such as a distribution transformer, to secure an electrical conductor to a housing of the device. Typically, an electrical conductor of an electrical bushing extends through an opening in a housing and is used to connect the internals of an electrical device to the outside world. A conventional electrical bushing includes an exterior insulating body having a mounting flange for securing the insulated electrical bushing to the exterior of a housing of an electrical device. A centrally-disposed electrical conductor is secured inside the insulating body and extends through an opening in the housing. An outer end of the electrical conductor protrudes from the exterior insulating body and is adapted for connection to an exterior connector device, such as an elbow connector. An inner end of the electrical conductor is connected to internal electrical components of the electrical device, such as windings, directly, or through a second electrical conductor.
An insulating body of an electrical bushing may be comprised of a ceramic material or a polymeric material. In many conventional electrical bushings composed of polymeric material, the electrical conductor is secured inside a pre-formed insulating body using an adhesive, O-rings and/or heat shrink tubing. An example of such a conventional insulated electrical bushing is disclosed in U.S. Pat. No. 6,515,232 to Forster. In the Forster patent, a conductor is disposed in a pre-formed insulating body comprised of glass-reinforced epoxy or a silica-filled cycloaliphatic resin system. Asphalt is poured between the conductor and the insulating body and O-rings and spring retaining gaskets are disposed at the top and bottom ends of the insulating body.
Some conventional insulated electrical bushings are formed in a much simpler manner by molding an insulating body directly over a conductor, such as is disclosed in U.S. Pat. No. 4,965,407 to Hamm and U.S. Pat. No. 5,281,767 to West et al. In the West et al. patent, the conductor is sandblasted before it is molded into the insulating body.
Although molding an insulating body directly over a conductor is much simpler than using O-rings and gaskets, the seal formed by a conventional over molding process tends to be less robust and often still requires the use of an adhesive.
It would therefore be desirable, to provide an electrical bushing that is simple to manufacture and has a robust seal. The present invention is directed to such an electrical bushing and a method for manufacturing the same.